Induction annealing as a method for expanded hydroformed tube formability

ABSTRACT

A method of hydroforming a workpiece includes the steps of bending the workpiece into a first preliminary shape, pre-forming the workpiece into a second preliminary shape, induction annealing the workpiece at a temperature between 120-160° C. and hydroforming the workpiece to a desired shape.

TECHNICAL FIELD

This document relates generally to the metal forming field and, more particularly, to a method for hydroforming a workpiece utilizing induction annealing to provide for enhanced formability.

BACKGROUND

Hydroforming is a term applied to sheet and tube forming in which the metal is formed against a die by fluid pressure. This may be done with an internal fluid pressure, with an applied axial load to a tube or with a one-sided die in which the sheet metal is formed by a bladder/diaphragm. Hydroforming typically uses conventional, single action hydraulic presses with high ram forces.

When using a multiple stage approach to hydroforming various bending/pre-forming steps may occur prior to the final hydroforming of the part. During these initial deformation stages most of the material formability can be exhausted leaving little plastic strain capability for the hydroforming process itself. One way to mitigate the influence of these localized strains is to apply a recovery heat treatment between stages of the hydroforming process. More specifically, induction annealing allows for local heating of a workpiece to a specified temperature in a specified time using an induction coil.

In one example, hydroformed aluminum tubes are used to form the A-pillar roof rail of a vehicle structure. This type of part may be made with structural or seamless extruded tubes. Structural tubes have better wall and diameter dimensional tolerances and are more efficient to extrude but have lower formability for bending, pre-forming, and hydroforming processes. As a consequence, structural tubes can only be used to form less challenging part shapes in hydroforming. Seamless tubes are less efficient to extrude relative to structural tubes due to scrap losses and seamless press cycle time limitations and have dimensional tolerances on wall and diameter that can be at least two time that of structural round tube. Seamless tubes can also have significantly higher formability which has made them the preferred material for hydroforming parts having complex, variable cross-sections such as A-pillar roof rails. In accordance with our work, it has now been discovered that induction annealing recovers sufficient workpiece formability to allow the less formable structural tubes in the production of A-pillar roof rails. Surprisingly, the formability may be recovered without compromising subsequent heat treatment to strengthen the roof rails.

SUMMARY

In accordance with the purposes and benefits described herein, a method of hydroforming a workpiece is provided. That method includes the steps of bending the workpiece into a first preliminary shape, pre-forming the workpiece into a second preliminary shape, induction annealing the workpiece at a temperature between 120-160° C. and hydroforming the workpiece to a desired shape. The method may also include ramping to the induction annealing temperature in 10-30 seconds. Further the method may include completing the induction annealing between the pre-forming and the hydroforming steps. In an alternative embodiment the method includes completing the induction annealing between the pre-bending and the pre-forming. In yet another embodiment, the induction annealing is completed between the bending and preforming steps and between the pre-forming and hydroforming steps.

The method may further include trimming the workpiece to desired length. Further the method may include heat treating the workpiece after hydroforming in order to impart desired T6 strength properties. That heat treating may be completed at 160-200° C. for 4 to 10 hours.

The workpiece may be made from AA6XXX aluminum alloy such as AA6082-T4. Further the workpiece may be an A-pillar roof rail. Accordingly the method may include completing the induction annealing to the A-pillar roof rail at a first bend of an A-pillar portion of the workpiece. That induction annealing may be completed (a) at a temperature of 120-160° C. with a ramp of 20 to 30 seconds, (b) at a temperature of 135-145° C. with a ramp of 20 to 30 seconds or (c) at a temperature of about 140° C. with a ramp of 25 to 30 seconds.

Stated another way a method of hydroforming a workpiece comprises: (a) bending the workpiece into a first preliminary shape; (b) pre-forming the workpiece into a second preliminary shape; (c) induction annealing the workpiece at a temperature between 120-160° C. to improve total elongation of the workpiece while not compromising final strength properties of the workpiece; (d) hydroforming the workpiece to a desired shape; and (e) heat treating the workpiece at about 180° C. for about six hours to improve final strength properties of the workpiece. In one possible embodiment the method may include using an induction annealing temperature of about 140° C. with a ramp time of 20-30 seconds.

These and other embodiments of the present method will be set forth in the description which follows, and in part will become apparent to those of ordinary skill in the art by reference to the following description of the method and referenced drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of the specification, illustrate several aspects of the present method and together with the description serve to explain certain principles thereof. In the drawings:

FIG. 1 is a perspective view illustrating a workpiece received in a channel coil for purposes of induction annealing.

FIG. 2 is a top plan view of the workpiece illustrating an A-frame roof rail and the A-pillar portion of the A-pillar roof rail subjected to induction annealing.

FIGS. 3 a, 3 b and 3 c illustrate three alternative hydroforming production processes incorporating induction annealing: in the first the induction annealing takes place between the bending and pre-forming steps, in the second the induction annealing takes place between the pre-forming and hydroforming steps, while in the third the induction annealing takes place between the bending and preforming steps and between the pre-forming and hydroforming steps.

FIG. 4 is a graphical illustration of T4 total elongation before and after induction annealing of tensile samples cut from extruded tube and subject to pre-straining. Such samples behave similarly to and are representative of A-pillar roof rail response.

FIG. 5 is a graphic illustration of T6-Temper verification confirming that A-pillar roof rails undergoing induction annealing still reach their desired yield strength after T6 heat treatment.

Reference will now be made in detail to the present preferred embodiments of the method.

DETAILED DESCRIPTION

This document relates to a method of hydroforming a workpiece W which incorporates an induction annealing step to enhance formability of the workpiece. In the embodiment illustrated in FIG. 1, the workpiece W is an A-pillar roof rail made from AA6XXX aluminum alloy such as AA6082-T4. As illustrated in FIG. 1, the workpiece W is positioned in a water-cooled copper “channel” coil C that follows the shape of the workpiece W. More specifically, alternating current flows through the channel coil C (transformer primary) to create an electromagnetic alternating field. The workpiece W forms the transformer secondary. Heating efficiency and uniformity is achieved through coil design and contouring of the coil relative to the workpiece shape.

FIG. 2 illustrates the workpiece W including the A-pillar portion P where induction annealing is completed from point A to point B. Portion T indicates the part of the workpiece W that is trimmed during the process as will be described in greater detail below. Note the roof rail portion R of the workpiece W may be of different lengths depending upon the body style of the vehicle (e.g. regular cab, extended cab, crew cab).

In one embodiment, induction annealing is completed at a temperature of between 120-160° C. utilizing a 10-30 second ramp time. In another useful embodiment, induction annealing is completed at a temperature of 130-150° C. utilizing a ramp time of 20-30 seconds. In another useful embodiment, induction annealing is completed at a temperature of 135-145° C. utilizing a ramp time of 20-30 seconds. In yet another useful embodiment, induction annealing is completed at a temperature of about 140° C. utilizing a ramp time of 25-30 seconds.

A first embodiment of a method of hydroforming a workpiece W is illustrated in FIG. 3 a. In the illustrated embodiment, the workpiece W comprises an extruded aluminum tube but it should be appreciated that the workpiece may assume other forms or be made from other metals. The method includes bending of the workpiece W into a first preliminary shape (see step 14). This is followed by the induction annealing of the workpiece W in the manner previously described (see step 16). The induction annealing process 16 utilizes current to locally heat the workpiece W in order to alleviate excessive strain hardening within the workpiece thereby allowing for increased formability during later stages of the hydroforming method.

This is then followed by the pre-forming of the workpiece W into a second preliminary shape (note step 18). This is then followed by hydroforming the workpiece W to a desired final shape (note step 20). Subsequent to hydroforming, the workpiece W is subjected to trimming to a desired length (note step 22). Following trimming the workpiece W is subjected to a heat treatment in order to impart desired strength properties to the workpiece W (note step 24). In the illustrated embodiment the heat treatment is a T6 treatment at 180° C. for six hours in order to induce or impart an average yield strength of typically 290 MPa to the workpiece W. In alternative embodiments the heat treatment may be completed at temperatures between 160-200° C. for 4 to 10 hours.

In an alternative embodiment of the production method illustrated in FIG. 3 b, the production process includes bending 28 the workpiece W into a first preliminary shape. This is then followed by the pre-forming 30 of the workpiece W into a second preliminary shape. Next the workpiece W is subjected to the induction annealing step 32 to recover formability. This is then followed by the hydroforming 34, trimming 36 and heat treating 38 steps previously described.

In yet another alternative embodiment illustrated in FIG. 3 c, the production process includes bending 40 the workpiece W into a first preliminary shape. This is followed by the induction annealing 42 of the workpiece W. Next the workpiece W is subjected to pre-forming 44 into a second preliminary shape. The workpiece W is then subjected to another incremental induction anneal 46. This is followed by hydroforming 48 the workpiece into a desired final shape. Subsequently, the workpiece is trimmed 50 and then subjected to artificial aging by means of heat treatment 52.

Any of the production method embodiments illustrated in FIGS. 3 a-3 c are particularly useful in the production of pickup truck roof rails which undergo significant bending of about 45° at the A-pillar portion P (see FIG. 2) in order to support the transition of the workpiece W from the A-pillar into the hinge pillar.

The data presented in FIGS. 4 and 5 shows that the induction annealing step or stage 16, 32 allows for an increase in material total elongation (strain) when used (note “IA” parts and also note that “weld” and “no weld” identifies where the tensile sample was cut from the structural tube). In fact upward of double the initial material strain capability is provided in high strain regions. Additionally, and surprisingly, no degradation to T6 Temper or heat treatment properties is observed when induction annealing is completed at temperatures between 120-160° C. for 10-30 seconds. Thus, total elongation is increased while the yield strength of the final workpiece product remains uncompromised. Such a combination of beneficial results could not have been anticipated.

In summary, numerous benefits result from the method 10 of hydroforming of a workpiece W as disclosed herein. Advantageously the method supports high volume automotive manufacturing. Both structural and seamless tubes benefit from the method. In fact, structural tubes may now be readily used in the production of difficult-to-form A-pillar roof rails. Thus, the method allows for the use of a higher tolerance and more manufacturing efficient material for hydroforming roof rails.

As should be appreciated the induction annealing process is restricted to the heated region of interest only: that is, the area of bending where plastic strain capability has been reduced by the bending and/or pre-forming steps or stages 14, 18, 28, 30 of the production process. Heating of the tube is localized to the induction annealed region, therefore, there is no specialized equipment required for material handling of the workpiece W in the unheated regions. Further the induction annealing parameters required to restore formability to the workpiece W do not cause post-hydroformed material heat treatment response damage and the formed workpieces (in the illustrated embodiment, A-pillar roof rails), are still able to demonstrate the desired yield strengths.

The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. For example, an incremental induction annealing step may be completed between both (a) the bending and pre-forming step and (b) the pre-forming and hydroforming steps if desired or at other times during production. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. 

What is claimed:
 1. A method of hydroforming a workpiece, comprising: bending said workpiece into a first preliminary shape; pre-forming said workpiece into a second preliminary shape; induction annealing said workpiece at a temperature between 120-160° C.; and hydroforming said workpiece to a desired shape.
 2. The method of claim 1, including ramping to said induction annealing temperature in 10-30 seconds.
 3. The method of claim 2, including completing said induction annealing between the pre-forming and the hydroforming.
 4. The method of claim 2, including completing said induction annealing between the bending and the pre-forming.
 5. The method of claim 2, including trimming said workpiece to a desired length.
 6. The method of claim 5, including heat treating said workpiece after hydroforming in order to impart desired strength properties.
 7. The method of claim 6, including completing said heat treating at 160-200° C. for 4 to 10 hours.
 8. The method of claim 7, wherein said workpiece is made from AA6XXX aluminum alloy.
 9. The method of claim 8, wherein said workpiece is an A-pillar roof rail.
 10. The method of claim 9, including completing said induction annealing to said A-pillar roof rail at a first bend of an A-pillar portion of said workpiece.
 11. The method of claim 10, including completing said induction annealing at a temperature of between 130-150° C.
 12. The method of claim 11 including ramping to said induction annealing temperature in 20 to 30 seconds.
 13. The method of claim 10, including completing said induction annealing at a temperature of 135-145° C.
 14. The method of claim 13 including ramping to said induction annealing temperature in 20 to 30 seconds.
 15. The method of claim 2, including completing said induction annealing at a temperature of 140° C.
 16. The method of claim 13 including ramping to said induction annealing temperature in 25 to 30 seconds.
 17. The method of claim 16, wherein said workpiece is made from AA6082-T4 aluminum alloy.
 18. The method of claim 17, including completing said induction annealing between both (a) the bending and pre-forming and (b) the pre-forming and hydroforming.
 19. A method of hydroforming a workpiece, comprising: pre-bending said workpiece into a first preliminary shape; pre-forming said workpiece into a second preliminary shape; induction annealing said workpiece at a temperature between 120-160° C. to improve formability of said workpiece while not compromising final strength properties of said workpiece; hydroforming said workpiece to a desired shape; and heat treating said workpiece at 160-200° C. for about 4 to 10 hours to increase final strength properties of said workpiece.
 20. The method of claim 19, including using an induction annealing temperature of about 140° C. with a ramp time of 20-30 seconds. 